1. Technical Field
The present invention relates to a virtual image display apparatus which presents a video formed by an image display element or the like to an observer, and particularly relates to a virtual image display apparatus which is appropriate to a head-mounted display which is mounted on the head of the observer.
2. Related Art
Various technologies have been suggested as an optical system which is embedded in a virtual image display apparatus, such as a head-mounted display (hereinafter, referred to as an HMD), which is mounted on the head of an observer (refer to Japanese Patent No. 3486468, Japanese Patent No. 3537230, Japanese Patent No. 3599828, Japanese Patent No. 4225606, Japanese Patent No. 4331290, JP-A-10-68887, and JP-A-2001-166209).
It is desirable that an apparatus weight be reduced for the virtual image display apparatus, such as the HMD. Since it is advantageous to bend an optical path by using a reflecting surface in order to make the optical system small and reduce the weight, various optical systems which use a prism have been developed. At this time, it is important that the reflecting surface or a refracting surface is a curved surface which is asymmetrical with respect to an axis and an effective aberration correction is performed by using shape flexibility of the curved surface.
For example, in Japanese Patent No. 3486468, Japanese Patent No. 3537230, and Japanese Patent No. 3599828, an optical system, which uses one prism, is illustrated. This type of optical system has an extremely simple configuration and is advantageous in reducing the weight because only one prism is used. However, there are only two reflecting surfaces which are available for the aberration correction in one prism, and thus, the shape flexibility is small, and an optical performance is not sufficient.
In Japanese Patent No. 4225606 and Japanese Patent No. 4331290, an imaging optical system which uses two prisms that is employed in a video camera or the like is illustrated. In the related art, it is known that the imaging optical system can be used as a display optical system which makes a virtual image when a forward direction of a ray of light is reversed into a backward direction. However, in the exemplified imaging optical system, an iris is provided between the two prisms, and cannot be used as the display optical system for the HMD.
In JP-A-10-68887 and JP-A-2001-166209, an optical system, in which two prisms are used and the iris further is disposed on a side of an object of a first prism, is illustrated.
However, in the exemplified imaging optical system and the display optical system for the HMD, specifications, such as a position of the pupil or a size of a display element are different from each other, and it is not easy to directly apply the systems. If the imaging optical system having a configuration in the above-described JP-A-10-68887 and JP-A-2001-166209 is directly applied as the display optical system for the HMD, since a distance from the prism which constitutes the display optical system to the position of the pupil is required to be 10 mm or greater, the prism is extremely large, and is not practical as a small-sized HMD.